There is no established standard governing allowable water content for turbine oil. However, in ASTM D4378-08 (Standard Practice for In-Service Monitoring of Mineral Turbine Oils for Steam and Gas Turbines), it is briefly mentioned that 0.1 percent or 1,000 parts per million (ppm) of water is considered a significant volume and should be condemned.
Indeed, 0.1 percent or 1,000 ppm is a large amount of water for a turbine system. Most turbine manufacturers would say this is unacceptable. Turbines may continue to run at these elevated moisture levels, but damage to the lubricant and machine will occur.
Water Content in Turbine Oil
In oil, water exists in three forms: dissolved, free and emulsified. The dissolved state occurs naturally and is not noticeable to the naked eye. Once the oil reaches its saturation point (the point at which it can hold no more water), the dissolved state of the water will become free or emulsified. Some free water will fall to the bottom of the sump due to gravity, while other water will be introduced to internal components. Emulsified water will appear cloudy or white and will impair film strength.
Before testing for the amount of water present in the oil, you can check to see if moisture is present at all using the crackle test. While this test will not tell you the amount of water in an oil, it is simple and useful for determining if further testing is needed. Tests like these are important to maintain healthy lubricants and machines and are learned through proper lubrication training.
Turbine Oil Water Limit
Some turbine manufacturers have defined 500 ppm or 0.05 percent as warning limits. A good strategy is to establish cautionary and critical moisture limits using the Karl Fischer test method. This method doesn't distinguish between the three states of water.
Machine criticality will also need to be evaluated. When reviewing criticality, consider the safety risks upon failure, the cost and length of downtime, the material and labor costs to repair, and the early warning systems. The higher the liability, the lower the target dryness should be set.
A better approach to selecting moisture limits based on machine criticality would be to employ the tactic of “as low as reasonably achievable” (ALARA). This takes the logic of measuring the moisture content a step further to exclude, remove or monitor water contamination.
Some turbine oils are formulated to resist the effects of water contamination with the help of additives better than others.
Keeping Moisture Out of Turbine Oil
Proper lubrication practices are key to maintaining turbine oil. Simpler devices like desiccant breathers do an excellent job of preventing humid air from entering the reservoir. Bottom sediment and water bowls can show free water that has settled to the bottom of the reservoir. If your budget allows or if your needs are critical, purchasing a vacuum dehydrator may even be warranted.
The best option is to keep the moisture content below the saturation point of the in-service temperature. The higher the operating temperature, the lower the oil's saturation point. Keep in mind that the fluid's age and the contaminants in the system will also have an impact on the saturation point.
